1. Field of the Invention
The present invention relates to an adjustable diameter pulley for use in belt transmission systems and particularly in serial printers.
2. General Description of the Prior Art
It is known that serial printers have a printing carriage on which a printing head is mounted. The carriage slides along two guide bars so as to occupy successive printing positions along a printing line. Examples of such printers are described in U.S. Pat. Nos. 4,044,882, 3,970,183 and 4,229,114. The guide bars are mounted between two sides of the printer frame and movement of the carriage is accomplished by a motor, preferably a step motor, coupled to transmission means, generally and preferably a cog belt. The cog belt is stretched along the guide bars between a driving gear and a driven pulley. The driving gear is splined to the shaft of the motor which, in its turn, is fastened to a side of the printer frame. The driven pulley is fastened to the opposite side of the printer frame by means of a bracket whose position may be adjusted. The precise positioning of the carriage is fundamental in order to obtain a correct printing of the characters. The positioning precision has to be therefore secured by a positive transmission--i.e. a transmission free from slack. For this purpose cog belts are generally used. The cog belts at present on the market are made of rubber, which is stiffened by steel cores. Accordingly the remaining fundamental problem to be solved is the one relating to the slack elimination.
One prior art method for solving this problem is to mount the driven pulley on a bracket which, after having been suitably positioned for the desired slack, is fastened with screws to the printer frame. In order to correctly position the bracket, suitable fixtures are used during the assembling phase; such fixtures allow the bracket to be positioned by exerting a suitable and controlled tension on the belt and then to fasten such bracket. This type of adjustment presents the following disadvantages: once the positioning fixtures are removed, a certain constraint yielding may occur and further, such yielding may increase during the printer operation; therefore the slack elimination does not correspond exactly to the desired values.
Another type of adjustment uses a movable bracket on which a spring exerts a suitable force opposing the force exerted by the belt on the pulley. Such type of adjustment eliminates slack, but it introduces into the transmission system an elastic constraint which modifies the tension state of the belt under variable dynamic condition. In fact, during the acceleration phases of the printing carriage in the direction of the driven pulley, the motor torque and the carriage inertia involve an increase of tension exerted by the belt on the elastic constraint; in contrast, during the acceleration phases of the printing carriage in the opposite direction, there is a reduction of tension. Therefore the above adjustment has the disadvantage of introducing a displacement between the real position and the ideal position of the printing carriage. Such ideal position of the printing carriage corresponds to a predetermined angular position of the driving gear. In addition to the above disadvantages oscillations may appear during the carriage movement. In order to reduce these disadvantages, it is necessary to exert a considerable stretching action. However, in such case, the belt tends to yield and wear, due to the considerable tension to which it is subjected and the position becomes less precise with time.
The present invention avoids all these disadvantages and permits the accurate adjustment of the belt tension and for eliminating slack, without requiring the use of special fixtures and without introducing elastic constraint and construction complications. Moreover, the present invention has the further advantage of permitting easy adjustment of the belt tension during field maintenance operations.